Miniature motor

ABSTRACT

The micromotor serves to drive an impeller ( 12 ) rotating in a pump housing ( 14 ). The excitation winding of the micromotor is surrounded by an enveloping flux return structure ( 18 ) of ferromagnetic material divided into rings ( 35 ). The rings ( 35 ) are separated from each other by slots ( 25 ). The slots are produced by laser cutting of a continuous tube. The enveloping flux return structure has a small wall thickness of a few tenths of a millimeter. The rings ( 35 ) are mutually connected by bridges ( 26 ). The enveloping flux return structure ( 18 ) can be integrally formed with the pump housing ( 14 ) of the pump ( 11 ). The micromotor can be produced in a small format with a small diameter. It has a high flow rate at a correspondingly high rotational speed. The micromotor is particularly suitable for introducing blood pumps into the vascular system in a non-operative minimally invasive percutaneous manner.

The invention relates to a micromotor with a stator comprising a sleevewith an excitation winding and an enveloping flux return structure ofspaced ferromagnetic rings, and particularly a micromotor combined witha pump portion and forming an impeller pump.

In the field of medical technique, blood pumps are needed which areintroduced into the body of a patient and placed in an artery or theheart to support the heart function. An intracardiac blood pump providedfor operation in the heart is described in DE 198 21 307. The blood pumpcomprises a drive portion wilth an electric micromotor and a pumpportion. It has a maximum outer diameter of about 8 mm and a rigidlength of less than 35 mm. The small-scale blood pump having therequired flow rate requires a high rotational pump speed of at least30,000 revolutions per minute, typically of 60,000 revolutions perminute.

A micromotor suitable for the operation of a blood pump is described inWO 98/44619. This micromotor has a stator and a permanently magnetizedrotor. The stator includes an excitation winding surrounded by anenveloping flux return structure. The enveloping flux return structurebundles the magnetic field lines generated by the excitation winding andconcentrates them so that only little magnetic flux is lost by strayfields. The enveloping flux return structure consists of pieces of sheetmetal which are electrically insulated with respect to each other andassembled to a tubular stack.

The wish to introduce pumps with a catheter into the vascular system ofa patient by punction without being forced to operatively open thepatient body results in the need of ever smaller pumps and motors thediameters of which are in the range of 4 mm. With such small dimensions,there is only very little room available for the enveloping flux returnstructure in the case of an electric micromotor. Here, the envelopingflux return structure can only have a wall thickness of a few tenths ofa millimeter. An enveloping flux return structure of such a small wallthickness cannot be assembled from pieces of sheet metal. On the otherhand, an enveloping flux return structure is useful in order to avoid aloss of energy by magnetic field leakage.

It is the object of the invention to provide a micromotor with highefficiency and small radial dimensions. This object is solved, accordingto the invention, with the features indicated in claim 1. Accordingly,the enveloping flux return structure consists of an integral bodywherein adjacent rings are connected by at least one bridge. This meansthat the enveloping flux return structure is made of an integral tube,and the rings are not completely electrically insulated with respect toeach other as is the case with sheet metal pieces. They are ratherinterconnected, on the one hand, and spaced, on the other hand, by thebridges. It is required to divide the enveloping flux return structureinto rings to limit the formation of eddy currents. Conventionally, therings are arranged separately from each other and completelyelectrically separated from each other by insulating layers. In the caseof the micromotor according to the invention, the rings are notcompletely separated from each other, the bridges, however, which areonly arranged at some locations of the periphery, do not lead to theformation of substantial eddy currents. Consequently, the entireenveloping flux return structure can be made of a continuous body. Theenveloping flux return structure can have a wall thickness of 0.2-0.4mm, i.e., an extremely small wall thickness, the bridges interconnectingthe rings providing the required cohesion and the necessary strength.Due to the small wall thickness of the enveloping flux return structure,it may occur that the enveloping flux return structure is operated inmagnetic saturation without being able to receive the entire magneticflux. The micromotor according to the invention has a rotational speedof at least 30,000 revolutions per minute and typically of 60,000revolutions per minute. The enveloping flux return structure has therequired structural strength required for the assembly as well as forthe operation of the micromotor. If thin lamellae were glued together,such a structural strength could not be achieved.

According to a preferred development of the invention, the bridges ofthe enveloping flux return structure form a longitudinal continuous web.This means that the bridges are oriented relative to each other. Thisorientation can be effected along a line parallel to the motor axis orpreferably along a helical line. Preferably, there are at least two webswhich are distributed around the circumference.

According to a preferred development of the invention, the envelopingflux return structure integrally passes into a pump housing. Theenveloping flux return structure divided into rings in the region of themicromotor is continuous in the region of the pump housing. Because ofthe integrity of pump housing and enveloping flux return structure, aparticularly exact centering of the pump housing relative to themicromotor is achieved. The assembly and the demands of precision madethereon are simplified.

Further, the invention relates to a method of manufacturing amicromotor. In this method, the enveloping flux return structure of themicromotor is manufactured of a ferromagnetic tube by laser cutting, inthe course of which peripheral slots interrupted by bridges aregenerated. Hereinafter, an embodiment of the invention is explained indetail with respect to the drawings.

In the Figures:

FIG. 1 is a side view of a pump with micromotor,

FIG. 2 is a longitudinal cross-sectional view of FIG. 1, and

FIG. 3 shows a cross-section along the line III—III of FIG. 2.

The pump illustrated in the figures comprises a micromotor 10 axiallyfollowed by a pump portion 11. The pump portion 11 comprises an impeller12 mounted on a shaft 13 and rotating in a tubular pump housing 14.

The micromotor 10 includes a stator 15 and a rotor 16 connected to theshaft 13. The stator 15 consists of a tubular sleeve 17 and anenveloping flux return structure 18 closely surrounding the sleeve 17.The rotor 16 includes a magnet 19 whose north pole N and south pole 1are arranged at diametrically opposed locations of the periphery. Themagnet 19 is fastened on the shaft 13. At the rear end, the shaft 13 issupported in the sleeve 17 by means of a ball bearing 20 and at thefront end facing the impeller 12, it is supported in a sealing bearing21.

The rear end of the stator 15 is followed by a transition piece 22adapted to be connected to a catheter (not illustrated). Wires 23connected to an excitation winding 24 in the interior of the sleeveextend through the transition piece 22. The excitation winding 24 isflown through by an externally controlled alternating current thefrequency of which determines the rotational speed of the motor. Thesleeve 17 including the excitation winding 24 consists of a plasticlayer with embedded wires and a thickness of about 0.2 mm. The wires arewound in two layers corresponding to a given configuration. Between thestator 15 and the rotor 16, there is a narrow gap in the dimensionalrange of one tenth of a millimeter.

The enveloping flux return structure 18 consists of an integral tubularbody into which circumferential slots 25 are cut. The slots 25 definerespective rings 35. Each slot 25 extends over less than 360°. In thepresent case, each slot is interrupted by two diametrically opposedbridges 26. The bridges 26 of adjacent slots form a longitudinalcontinuous web 27. Here, this web extends helically. A corresponding web27 is located on the opposite side not illustrated in FIG. 1.

The slots 25 connecting adjacent rings 35 are made by laser-cutting of aferromagnetic tube, the cutting width being less than a tenth of amillimeter, particularly about 0.05 mm. Generally, the number of slots25 should be as great as possible so that the rings are as narrow aspossible. Preferably, the width of the rings is 0.2-0.3 mm.

The slots 25 are filled up with plastic material 28 (FIG. 3), resultingin a continuous smooth outer surface of the enveloping flux returnstructure 18. At the same time, the enveloping flux return structure 18forms the outer skin of the micromotor. If necessary, it can be coatedwith an additional plastic layer.

As shown in FIG. 3, the slots 25 filled with the plastic material 28extend through the entire thickness of the enveloping flux returnstructure. The enveloping flux return structure 18 closely surrounds thesleeve 17 containing the excitation winding 24.

The wall thickness of the enveloping flux return structure amounts toabout 0.25 mm, the outer diameter amounts to 4 mm and the inner diameter3.45 mm. The length of the enveloping flux return structure 18 is 12 mm.

The enveloping flux return structure 18 is continued by webs 30forwardly projecting from the front end of the micromotor and integrallypassing into the wall of the pump housing 14. Therefore, the pumphousing 14 is integrally formed with the enveloping flux returnstructure 18 and consists of the same material. This means that the pumphousing 14 has the same outer diameter and the same inner diameter asthe enveloping flux return structure 18. In the present embodiment, thewebs 30 do not extend parallel to the axis of the micromotor, buthelically, in correspondence with the course of the flow generated bythe helical impeller 12. When the pump housing is formed integrally withthe enveloping flux return structure, the wall thickness in the regionof the pump housing is alternatively smaller than in the region of theenveloping flux return structure. Thus, a uniform tube can be providedthe inner diameter of which is made larger in the region of the pumphousing than in the region of the enveloping flux return structure bydrilling or boring, while the outer diameter is the same all over.

The openings 31 formed between the webs 30 form the outlets of the pumpand the end opening 32 of the pump housing 14 forms the inlet of thepump. The pump can also be driven in opposite direction so that theopenings 31 form the inlet and the opening 32 forms the outlet.

What is claimed is:
 1. A micromotor comprising a stator including asleeve with an excitation winding and an enveloping flux returnstructure, wherein said enveloping return structure comprises anintegral body formed from a single piece of sheet metal having spacedferromagnetic rings wherein adjacent rings are connected by at least onebridge.
 2. The micromotor of claim 1, wherein said at least one bridgeforms a continuous web along said enveloping flux return structure. 3.The micromotor of claim 1, wherein a pump housing comprises an integralportion of said enveloping flux return structure.
 4. The micromotor ofclaim 3, wherein spaces between said spaced ferromagnetic rings arefilled with a plastic material and said enveloping flux return structureforms an outer wall of a motor housing.
 5. The micromotor of claim 3,wherein said enveloping flux return structure has an outer diameter ofnot more than 4 mm and an inner diameter least 3.3 mm.
 6. The micromotorof claim 2, wherein a pump housing comprises an integral portion of saidenveloping flux return structure.
 7. The micromotor of claim w, hereinspaces between said spaced ferromagnetic rings are filled with a plasticmaterial and said enveloping flux return structure forms an outer wallof a motor housing.
 8. The micromotor of claim 6, wherein saidcontinuous web defines a helix.
 9. The micromotor of claim 2, whereinspaces between said spaced ferromagnetic rings are filled with a plasticmaterial and said enveloping flux return structure forms an outer wallof a motor housing.
 10. The micromotor of claim 9, wherein saidcontinuous web defines a helix.
 11. The micromotor of claim 1, whereinspaces between said spaced ferromagnetic rings are filled with a plasticmaterial and said enveloping flux return structure forms an outer wallof a motor housing.
 12. The micromotor of claim 11, wherein saidenveloping flux return structure has an outer diameter of not more than4 mm and an inner diameter of at least 3.3 mm.
 13. The micromotor ofclaim 1, wherein said enveloping flux return structure has an outerdiameter of not more than 4 mm and an inner diameter of at least 3.3 mm.